328
M. K. RERNETT,N. L. JARVIS AND W. A. ZISMAN
For the low values of W/g,the molecules of ethyl chloride occupy the sites of highest binding energy, and the dipole relaxation time is much larger than the phase interval between reversals of the field. Thus COT is very much greater than 1, and df is immeasurably small. Almost until the completion of the monolayer, c f f remains small. Near the completion of the monolayer, sites of lower binding energy begin to be occupied, and 7 becomes smaller so that COTapproaches unity. Thus a measurable absorption region arises, e l f increasing as amount adsorbed increases because T decreases. This continues until the monolayer is completed. Because of the difference in heat of adsorption for the first and subsequent layers, a rather sharp decrease in relaxation time is to be expected a t monolayer completion. (Perhaps even the mechanism of orientation changes.) For such a sudden decrease, COT becomes much less than 1. Thus amounts adsorbed past the monolayer will not contribute appreciably to e", but those molecules already adsorbed before completion of the monolayer retain their characteristic relaxation times,
Vol. 66
producing a plateau in the curve for amounts adsorbed past the monolayer. This is in contrast to the alumina-water system in which Baldwin2 found that e" did not begin to increase until after the monolayer had been completed. The studies of McIntosh and co-worker~~--~ of ethyl chloride on silica gel and on rutile were made in the megacycle region and thus cannot be compared with the alumina-ethyl chloride system. The values of V m as found by identifying the point of monolayer completion from the plateau characteristics are in good agreement with those found from the adsorption isotherm. At a frequency of 1 kc., V , = 51 mg./g.; a t 10 kc., V m = 50 mg./g.; and at 50 kc., V , = 49 mg./g. Thus it seems reasonable to identify the beginning of the plateau in the 6'' os. amount adsorbed curve with the completion of the monolayer. The author wishes to thank Professor J. C. Morrow for his valuable suggestions and criticisms. The help of Mr. D. E. Sampson and Dr. M. G. Baldmin in the construction of the vacuum system and dielectric cell is gratefully acknowledged.
SURFACE ACTIVITY OF FLUORINATED ORGAXIC COMPOUNDS AT ORGANIC LIQUID-AIR INTERFACES. PART IV. EFFECT OF STRUCTURE AND HOMOLOGY1 BY MARIANNE K. BERNETT,N. LYNXJARVIS AND W. A. ZISMAN U.S. Naval Research Laboratorv, Washington 86,D. C . Received October 2, I961
Previous investigations have shown that partially fluorinated carboxylic esters adsorb a t organic liquid-air interfaces as monomolecular films, thus depressing the surface tension of the organic liquid. I n this investigation the surface activities of specially designed fluorinated solutes were studied in seven organic solvents of different compositions and surface tensions. B y varying the structure and composition of the organophobic the organophilic and the connecting polar groups, their contribution to solubility, adsorptivity, and orientation and pack& of the solute molecules a t the organic liquid-air interface could be investigated. The degree of solubility, as well as the packing of the molecules at the surface, was shown to be dependent upon fluorination, length and number of the organophobic chain, and the structure of the organophilic portion. From the force-area isotherms the lowest area per molecule attainable in each solvent was calculated. The relation of these lowest areas to the corresponding lowest values of surface tension obtained was discussed in terms of solute structure and orientation a t the interface. As indicated by surface tension values and the steep initial slopes of the surface tension us. concentration curves, several of the new fluorinated solutes have much higher surface activity than those previously reported.
Introduction I n Part I2of this investigation it was shown that the initial spreading coefficient can be used to rapidly screen a large number of amphipathic compounds for possible surface activity in organic liquids. By using this technique several classes of partially fluorinated organic compounds were shown qualitatively to be promising surface active agents for organic liquids. I n Part IIa it was demonstrated that the partially fluorinated carboxylic esters were surface active in various organic liquids, the surface tension depression in any one organic liquid being a function of the balance between the organophobic and organophilic constituents of the molecule. These fluoroester solutes adsorbed at the organic liquid-air interfaces as (1) Presented before the Division of Colloid and Surface Chemistry, American Chemical Society, a t the 140th National Meeting in Chicago, Illinois, September 4-8, 1961. (2) N. L. Jarvis and W. A. Zisman, J. Phv6. Chem., 68,727 (1959). (3) N. L. Jarvis and W. A. Zisman, ibid., 64, 150 (1960).
unimolecular films whose orientation and packing depended upon the molecular structure, solubility and extent of association of the solute and solvent molecules. From the F os. A isotherms, equations of state were calculated in Part 1114for each adsorbed monomolecular film. It was concluded that the adsorbed molecuIes fail to form close-packed condensed monolayers even a t the highest film pressures; a t low film pressures all films are gaseous monolayers. I n the present study emphasis is given to the search for fluorinated solutes having the highest possible surface activities in organic liquids. For this purpose selected types of partially fluorinated compounds were designed, synthesized and studied, some in homologous series. The previous studies suggested some primary qualifications for optimum surface activity, such as low volatility, organophobic fluorocarbon chains so situated as to present (4) N. L.Jarvis and W.A. Zisman, i b i d . , 64, 157 (1960).
Feb., 1962
SURFACE ACTIVITY OF FLUORINATED ORGANIC COMPOUNDB
clos,e packing on adsorption at the solvent surface, and organophilic constituents sufficiently soluble to impart desirable orientation of the molecule a t the interface. With these requirements in mind, as well as obvious requirements of reasonable oxidative and hydrolytic stability, compounds were designed in which straight chain or highly branched hydrocarbon groups, benzene rings or chlorinesubstituted benzene rings were used as the organophilic constituents of the molecules. TO study the effects of homology, the number or length of the orga,nophobic fluorocarbon chains in the molecule was varied. The effect of the replacement of the terminal fluorine atom of a fluorocarbon chain by hydrogen also was investigated. Since the organophobic and organophilic portions of the molecule may be connected by polar groups, which may be either organophilic or organophobic depending upon their structure and the nature of the substrate, a variety of these were included in thie study.
$-DERiVATlVES
38
329
I
IN NiTROMETHANE A
::I
& 30 .-
4 0 -
A 24
01
I
02
I
03
05
04
CONCENTRATION ( M O L E S / L I T E R
I.
Fig. l a
!
HEXADECANE
&
30
x
28-
I
Materials and Experimental Procedures A11 of the compounds used as surface active agents, with the exception of the @'-octyl alcohol, were synthesized for this investigation by J. G. O'Rear and P. J. Sniegoski of the Surface Chemistry Branch of this Laboratory. Each compound was a small scale preparation of high purity; the methods of reparation will be described in a subsequent publication gy O'Rear and Sniegoski. Table I lists the compounds investigated along with selected physical constants. Uising reviously established nomenclature,s .the perfluoro alcohof derivatives are denoted as @'-derivatives and w-monohydroperfluoro alcohol derivatives as $'-derivatives. Seven organic liquids with graded surface tensions and different chemical compositions were used as solvents. The source, method of purification, and essential physical constants of propylene carbonate ( y = 41.1 dynes/cm.), tricresyl phosphate ( y = 40.2 dynes/cm.), Alkazene 42 ( y =: 38.2 dynes/cm.), nitromethane ( y = 36.4 dynes/cm.) and hexadeicane ( y = 27.6 dynes/cm.) are given in Part 1.2 The additional organic liquids used in this study were dioxane ( y =: 32.9 dynes/cm.) and ethylbenzene ( y = 28.6 dynes/cm.), both of which were obtained from Eastman Organic Chemical Company. The dioxane was dried just before use with Linde Molecular Sieves. Solutions of the surface active agents were prepared and their surface tensions measured using the procedures described in Part 11.8 The same method of preparation was used for the solid fluorocompounds as for the liquids, except that in the former case they were weighed on a semimicro balance. 4.11 surface tension measurements were made a t 20 =z! 0.4" and 50% R.H.
A
22t20 0
I
,001
n
I
,002
I
,003
I
,004
I
,005
I
006
I
,007
I
I
,008
,009
.08
09
.01
CONCENTRATION !MOLES/LITER).
Fig. l b
ETHYL B E N Z E N E
38
;28 26
--Q--ao------
24 22
zoo
-01
.02
03
04
05
06
07
I
CONCENTRATiON (MOLES/LITER).
Fig. IC
DIOXANE
Results and Discussion Surface Tension Lowering and Solubility.Figures la-lh summarize the results of the sinface tension measurements made for each fluorinated solute in the various organic solvents. Due to !imiting solubility all the solutes were not studied in every solvent, the choice of solvent being predicated upon the structure of the respective fluorochemical. TLe derivatives of the $'-alcohols, compounds 5-8 ;and 10-11 of Table I, all had higher surface tensions than the derivatives of the-@'-alcohols, but they were also more soluble in the polar organic solvents studied. It was suggested in Part I1 that the terminal hydrogen of a #'-compound weakly associates with the polar oxygen group in ( 5 ) P. D. Faurote, C. M. Henderson, C. M. Murphy, J. G. O'Rear and H. Ravner, Ind. Eng. Chem., 48, 445 (1956).
CONCENTRATION !MOLES/LITERI.
Fig. Id
the more highly associating organic solvents, thus imparting high solubility to the #'-compound. As can be seen in Fig. l a this combination of high surface tension and high solubility resulted in very low surface activity for the $'-alcohol derivatives, m7ith the exception of tris-($'-nony1)-tricarballylate. This low surface activity is reflected
461 -
..
NITHOMETHANE
I
LO
26
- - _--o---
..
29 L C
L COi
_ _ .L __!__-GC-I
- GGB
24 -
-
421
-
~
3
0
0
4
0
0
5
0
0
6
0
0
7
0
1
TABLE I
Number
1
2 3 4
5 5
6 7 8 03
04
05
010
A c i w E COXIPOUKDS ( A T 20’)
T A I C R E S Y L PHOSPHATE
02
,
C09
h I Y S I C A L CONSTANTS O F PARTIALLY FLUORINATED SL‘RFACE
Fig. If
01
I
1
035
Fig. I h Fig. 1.-Surface tensions of partially fluorinated compounds in organic li uids: 1,tris-( $’-propyl)-tricarballyl~te; 2, tetrakis-($’-amJ)-butane tetracarboxylate; 3, bis-( $’hepty1)-phenyl succinate; 4, bis-( +’-hepty1)-tetrschlorophthalate; 5, his-( $’-hepty1)-phenyl glutarstc; 6, tris-( I)’nonyl)-tricarballylate. A, +’-octyl alcohol; A, +’-octyl ethanesulfonate; V, bis-(+’-oct.oxy)-bis-(t-biitoxy)-silane; V, tris-(+‘-octoxy)-t-butoxysilane; 0, bis-(+’-octy1)-tctrachlorophthalate; 0 , tris-(+’-butyl)-tricarballylate; 0 , txis(9’-octy1)-tricarballylate; D, bis-(+’-octy1)-dodecenyl succinate; c), bis-(6’-octyl)-toluene dicarbamate; 8,hexyl 6‘butyrate3; 8, 1,2,3-trimethylolpropane tri~-(+’-but,yrate)~; 8 , bis-(~’-hexyl)-3-methylglutarate3; 8 , bis-(+’-octyl)-3methylglutarate.3
CONCENTRATION (MOLESILITERL
0
,
0C5 006 007 CONCEYTSATION ! M O L E S / L I T E R I ,
3>0
003
I
b
!
---LI-i2 00L . - i0-0.1i - 2 -C.2!: - L . i -003 004
i_-
003 0 0 4 005 006 007 CONCENTRATION I M O L E S / L I T E Q l
O
--
2 2;-
Fig. l e
Ob2
1-
06
07
08
09
i
CONCENTRATIOL IMOLES/! i T € S ‘
9
Fig. l g
in the small values given in the sixth column of Table I1 for the initial slopes of the surfarc tension us. concentration curves in Fig. la. Due to the low surface activity of the dcriwtives of the $’alcohols, no study mas made of their behavior in solvents other t han nit romethane . The solubility of the 4’-compounds generally followed the pattern outlined in Part IT, where the solubility mas shown to bc a function of the balance bctween the organophobic arid organophilic constituents in the molecule. The polar groups used to join the fluorocarbon and hydrocarbon groups in the moleculc were organophilic to {he extent that they associated with the solvent molecules. Table I11 lists the estimated solubility values for the fluorocarbons obtaincd by extrapolating the surface
10 11
Surface active solute
+’-Octyl alcohol +’-Octyl ethanesulfonate Bis-(+’-octoxy)-bis-(t-butoxy )silane Tris-(@’-octoxy)-t-butoqsilnne nip-( $’-hepty1)-a-phenyl glutsrate Bis-( +’-hcpty1)-phenyl succinntc Tetrakis-( $’-smyl)-1,2,3,4butane tetracarbovylnte Bis-( $’-hepty1)-tetrachlorophthalate Bis-( 9’-octy1)-tetrachlorophthalate Tris-($’-prop3.l)-tricarballylnte Tris-($’-nony1)-tricarballylatc
12 Tris-(~’-hutyl)-tric~:irhnllylntc 13 Tris-(+’-octy1)-tricarhnllylat P 14 Bis-( +’-octyl)-a-n-tiodereriyl
succinate 15 Bis-(+’-oct yl)-2,4-t oluene dicarhsmate
g./nil.
Surface tension, dynes/ em.
Solid 1.709
19.1
1 525 1 702
18.1 18.5
1 6164 1 8100
26.2 25.9
Density,
..
Solid
..
Solid
..
..
Solid 1 1012 Solid
31 . o
1 813
18.5
Solid
..
1 484
10.4
Solid
..
..
tension os. concentration c u r x for each solut ion until it intersected a horizontal line represcntirig thc surface tension of the saturated solution. Thc contributions of the polar connecting groups to thc solubility in both polar and non-polar solvent s is made evident by the higher solubility of the tricarballylatcs in the associating solvcnt s thaii iii non-polar et hylbenzrnc. I n the cthylbcnacnc solu-
SERFACE ACTIVITY OI?FLTJORINATED ORGAKICCOMPOUNI)~
lW)., 1002
331
TABLE I1 I N I T I A L sl.Ol'E Ob' SUR17ACE ' h N S I O K V S . C O N C E N T R A T I O N C U R V E S (hT 20')
Surface ____ Initi\lslopeX-1!!2--__. - .-tension, y . I'ropylene Tiicreayl NitroICtliylIlexadynes/ carhonate phosphatr dlkascne methane 1)ioxanc bcnsene dccane cm. ( y = 41.1) ( y = 40.2) (y = 38.2) (y = 36.4) ( y = 32.9)( y = 28.6) (7 = 27.6) 0.56 ... .. 0.25 1.4 0.56 0.36 .. .21 ... . . 0.2 0.9 0.6 19.1 0.90 .80 ... .. 64 60 280 18.4 410 _____I_
Surface active solutc
+'-Octyl alcohol +'-Octyl ethanesulfonate Bis-(+'-octoxy )-his-( t-butoxy )-silane 18.5 Tris-( +'-octoxy)-t-butoxysilane 26.2 %s-( $'-hepty1)-a-phenyl g1utar:it e 25.9 nis-( $'-hepty1)-phenyl succinate Tetrakis-( $'-,zmyl)-l,2,3,&butane .. tetmcnrboxylatc Bis-( $'-hepty1)-tetrachlorophthalate .. Bis-( 6'-octy1)-tetrachlorophthahte 31.0 Tris-( $'-propyl)-tricnrballylute .. Tris-( $'-nony1)-tricarballylate 18.5 Tris-( +'-butyl)-tricarballyla te Tris-(d'-octSl)-tricarbsllylate Bis-(+'-octy1)-a-n-dodeccnvl succinate 10.4 .. Bis-(+'-octyl)-2,4-1,oluene dicarbnmntc
120
455
....
....
....
....
10.0
40
380
0.035 ,028
...
...
.018 ,036
...
11
72
.... ....
.... ....
...
0.004
...
.8 .56
2.6 110 30
.... ....
310 32
,..
...
..
1.6
0.20
... 0.2
..
, . .
,I2
...
..
14
.GO
4.8 0.56
.....
...
,.
, . .
T A ~ LI11 E ~ O l , ~ J ~ I l .O I lF~PYA R T I A l , I , Y F t V O R I N A T E n S U R F A C E .\C.TIVE COMPOLJK-DS I N O R O A K I C TdIQUlnS ( A T
Surface tension, 7.
Surface active solute +'-Oct.yl alcohol +'-Octyl cthanesulfonate Bis-(+'-octoxy)-bis-(f-butoxy)silane Tris-(9'-octoxy)-t-butoxysilane Bis-(0'-octy1)-tetrachlorophthalate Bis-(+'-oetyl) -a-n-dodecenyl succinate Bis-(~'-octyl)-2,4-tolucnedicarbarnata Tris-(+'-butyl)-tricarballylatc Tria-(0'-octyl)-tricarballylate Tris-(~'-nony1)-tricarballylate
dynes/ cm. Solid 19.1
Propylene carbonate ( y = 41.1) >0.1 >0.1
Solubility (moles/l.) of solute Tricresyl Alkaeene Nitrophosphate 42 niet,hsne Dioxane ( y = 40.2) ( y = 38.2) (y = 36.4) ( y = 32.9) >0.1 5.9 x lo-' >O.l >0.1 1.85 X 10-2 >0.1 2.3 X 10-4 7.9 X lo-' 4.9 x 1 0 3 2 X 10-6
Solid
1.7 X 10-4
4.3
19.4
3.5 x 10-4
2.93 x 10-8
Solid 18.5 Solid Solid
6.2 X 10-4 1 . 0 2 x 10-1 0.1
x 10-2 0.1 5.25 X 10-9 3.25 X 10-8 hsurfaco t'oiisions of 15-18 dyncs/cm.8j9 and aqucous systems coiit,airiing inixturcs of perfluoroijctyl alcohol and conventional wtt'ing agents Icachcd surface tciisioiis as low as 15.2 dyrics/cm.12 Thcsc low surface tensions correspond t'o rather close-packcd films of -C1:2- and -cI?3 groups at the water-air interface. As an indication of the surface frcc energy of a --CY2- saturated surface, Zisman arid co-workers foulid the critical surfac~ tension of polytetrafl~ioroo1,hylcno (primarily -CE'r-) to t)c 18.4 dyiics/cm.,'s while for a closdy packed --CE's surface it is about (i dynes/cm.16 The importmcc of orieiit n t h i of the fluorocarbon
havior in any region; at best a liqiiid condenscd film was formed, as with bis-(4'-octoxy)-bis-(lbut
